Iso-1-cytochrome c and iso-2-cytochrome c from the yeast Saccharomyces cerevisiae are two of the few proteins of known primary structure from a microorganism which is particularly suitable for experimental genetic studies. The isolation of appropriate mutants have been facilitated by enrichment procedures for both forward and reverse mutations. A series of deletions are available, making it possible to conveniently map point mutants and to estimate their positions relative to the iso-1-cytocrome c sequence. The large number of mutants that have been characterized and the selection procedures permit an unprecedented degree of genetic manipulation of nucleotide sequences by recombination. The DNA sequences of the structural genes of iso-1-cytochrome c (CYC1) and iso-2-cytochrome c (CYC7) as well as the adjacent regions have been determined. Thus the body of information concerning the iso-1-cytochrome c gene, the large number of defined mutants and the available genetic and biochemical techniques are without parallel for any other eukaryotic gene. The iso-cytochrome c system is being employed for investigation numerous problems in molecular biology and genetics, including: the identification of amino acids inserted by various nonsense suppressors; the determination of the specific action of numerous mutagens; gene conversion studies and the examination of the relationship of recombination frequencies and nucleotide alterations; the isolation and characterization of structural and regulatory mutants for both iso-1 and iso-2-cytochrome c; the isolation and characterization of mutants overproducing iso-1-cytochrome c iso-2-cytochrome c; the characterization of DNAs and mRNAs specifying the two iso-cytochrome c and their measurements in normal and mutant strains; the determination of DNA sequences of mutations that are in the nontranslated regions of the CYC1 and CYC7 genes.